1. Field of the Invention
The present invention relates to the field of reclamation and reuse of semiconductor material substrates. More particularly this invention relates to an apparatus and method for removal of surface films on wafers at a reclaim factory.
2. Discussion of Related Art
The increasing process complexity and introduction of new materials to the field of integrated circuit (IC) fabrication has given rise to a greater number of processing steps; each of which must be tested for quality.
Test wafers including “dummy” or “control monitor” wafers are used to check the reliability of IC fabrication equipment. For example, dummy wafers are used to test new IC fabrication equipment prior to its implementation into the large-scale production process of ICs. A dummy wafer is cycled through the new equipment and the ICs formed on the dummy wafer are then examined to determine if they meet certain specified criteria indicating that the fabrication process was properly performed. Only then is the equipment implemented into the production of ICs. Thereafter, the dummy wafer may be discarded, or “recycled” by removing the deposited films and re-using the dummy wafer.
Once fabrication equipment is implemented into the production process, it must be periodically inspected by examining the fabricated ICs to ensure that it is functioning properly. Such quality assurance testing is typically performed on a daily basis, such as at the beginning of every working shift. During such testing, control monitor wafers are used in a trial process, such as film deposition, performed on the wafer. The control wafer is then examined to determine if it meets certain specified criteria indicating that the fabrication process was properly performed. Thereafter, the control wafer may be discarded (to protect intellectual property, for example), or “recycled” by removing the deposited films and re-using the control wafer.
All of this quality assurance testing requires the use of a large number of wafers and increases the total cost of IC fabrication. Customers will typically recycle their wafers using their own equipment. However, each recycle roughens the wafer surface and after a few such cycles the wafers must be re-polished to meet fab specifications for such wafers to be used in their tools. These wafers are typically sent to a wafer reclaim vendor who provides the essential expertise and service for stripping and re-polishing the wafers to the customer's specifications and returning them to the customer for a service charge.
The reclamation cycle forms a loop in which used wafers are sent to a reclaim vendor, processed to meet fab specification, and sent back to the customer for reuse as test wafers. Customers optimize cost-cutting by reducing the number of test wafers to be used, and by using them as many times as possible. This requires maintaining a high ratio of reclaimed wafers to total test wafers. In order to meet customer demands, wafer reclaim factories must in turn optimize the wafer reclaim process and offer cycle times in the order of days rather than weeks.
A typical wafer reclamation process includes multiple preliminary steps of incoming wafer inspection, ID detection, and sorting of the wafers into groups. The grouped wafers are then subjected to removal steps such as grinding and/or etching particular materials, followed by polishing and cleaning. The process is finalized with a final multi-step outgoing wafer inspection to ensure that the proper amount of material was removed, and that customer specifications such as those for surface particles and wafer flatness are met.
The presence of copper films in the back-end processes has posed new problems to the wafer reclaim industry. Particularly, copper bulk contamination during the stripping process is detrimental because rapid diffusion of copper in silicon at relatively low temperatures can cause the metal to form deep level traps for carriers. It is risky to employ traditional film-removal methods, such as grinding and etching techniques because both have shortcomings when it comes to removing copper.
Traditional etching techniques involve the immersion of wafers in a series of chemical baths to strip film materials and etch the wafer. The chemical solutions used to remove films from wafers typically contain hydrofluoric acid (HF). Hydrofluoric acid solutions etch oxidized silicon, which can cause severe pitting of the wafer surface. Furthermore, any metallic contamination, such as copper, present in the stripping solutions from etching of the films has a very high likelihood of contaminating the bulk silicon.
Traditional grinding techniques physically remove films by placing a wafer between two counter-rotating plates and lapping away material. While this technique effectively removes films from the front and back surfaces of wafers, it does not remove contamination on the wafer edge. Wet grinding with slurries poses additional issues of particle contamination on opposite surfaces. On the other hand dry grinding without slurries, while having the benefit of reduced cross-contamination compared to wet grinding, causes severe microcracking that can extend between 10 μm and 50 μm into the substrate. This 10 to 50 μm damaged layer has to then be removed in subsequent polishing steps to recover the original wafer finish. Consequently, this results in the possibility to reprocess the wafer only a few times.
Therefore, what is needed is a reasonable and cost-effective method for removing wafer surface and edge films that does not suffer the negative impacts associated with current etching and grinding techniques.